Interaction between immunologically competent cells (ICC) and murine oncogenic viruses resulting in an alteration of immunological function, will be studied in experimental models developed previously in our laboratory. We found that from two populations of specific ICC in the spleen, anti-sheep erythrocyte antibody forming cells are suppressed by infection with Friend leukemia viru (FLV) and carry virus-induced antigens on their surface; in contrast, anti-cholera vibrio antibody forming cells do not have the virus-induced antigens and are normally competent. However, the latter become susceptible to FLV when pre- sensitized with cholera antigen before the virus infection. Secondly we found that the specific function of antigen binding is preserved in the population of ICC incapable of antibody synthesis. Whether there are other immune functions maintained is to be determined. Thirdly, immunosupressed ICC can return to immunocompetence when virus proliferation ceases. On the basis of this data, the following objectives were set for the proposed project: to determine the phase in which ICC are resistant to virus infection, and the mechanism of this resistance; to delineate the step of functional differentiation in which ICC become susceptible to and suppressed by the virus; to find which specific functions may be preserved in infected cells incapable of antibody synthesis; and to study the mechanism of conversion from immunosuppression to immunocompetence in vitro. An experimental plan with these objectives would elucidate the general problem of differentiation of immunocompetent cells as well as the mechanism of immunosppression in leukemic organisms. It would also contribute to the immunotherapeutical approch to virus-induced malignancy.